As more weapon and fire-control systems become dependent on GPS (Global Positioning System) for their accuracy and effectiveness, it becomes important for GPS receivers to be able to withstand RF (Radio Frequency) signal interferences, especially under a highly dynamic engagement scenario. The RF interference can adversely affect GPS receiver code and carrier tracking, resulting in degraded and unsatisfactory navigation performance.
In essence, a GPS system receiver determines its terrestrial location by triangulating its position relative to GPS satellites in orbit around the earth, by receiving signals transmitted from the satellites, measuring the travel times of the signals from the satellites to the receiver, and then calculating the distances of the satellites from the receiver based on the travel time. To measure the travel time, very accurate timing is necessary—the GPS satellites carry atomic clocks. The receiver also needs to know the exact positions of the GPS satellites. In addition, for further accuracy the receiver must compensate for atmospheric effects on travel time of the satellite signals to the receiver.
One basic function of a GPS receiver is to generate replica signals that can be correlated with the received satellite signals. Each GPS satellite can have a unique digital code sequence (e.g. a Pseudo Random Code) that is by analogy similar to a musical tune, so that the GPS receiver can distinguish signals from different GPS satellites. The GPS receiver knows the “tunes” or code sequences of the different GPS satellites, knows when the “tunes” are to be transmitted, and knows where the GPS satellites should be.
Upon receiving a code signal, the GPS receiver identifies the signal, generates a replica of the code signal, and seeks to “hum along” or synchronize the replica signal with the received code, and thereby track the received signal. This signal tracking includes two fundamental functions: code-phase tracking to track digital codes of received satellite signals, and carrier-phase tracking to track the carrier signals that are conveying the digital codes. When the receiver is receiving a code signal from a GPS satellite and the receiver's clock is synchronized with the clock onboard the satellite, then an amount of time that the receiver must delay the code replica signal to synchronize or correlate it with the received code signal, is the amount of time it takes the received signal to travel from the satellite to the receiver. The receiver can use this time interval to determine a geographic distance between the satellite and the receiver. Signals from four or more different GPS satellites enable the receiver to synchronize its clock with the clocks onboard the satellites.
GPS satellites operated by the U.S. military transmit two different signals on two different carrier frequencies. The first carrier frequency, L1, has a frequency of 1575.42 MHz and carries two pseudo random digital codes as well as a status message (containing, for example, supplemental information regarding the satellite's orbit, the accuracy of its clock, and so forth). The first digital code on L1 is called a C/A (Coarse Acquisition) code. The U.S. military makes the C/A code for each U.S. GPS satellite known and available to the public sector. The C/A code repeats every 1023 bits, and modulates the L1 carrier frequency at a 1 MHz rate. The second carrier frequency, L2, has a frequency of 1227.60 MHz. In addition to the C/A codes transmitted on the L1 carrier frequency of the U.S. GPS satellites, a P(Y) code (“P” for precise, and “(Y)” when the code is encrypted) is also broadcast from each satellite on both the L1 and L2 carrier frequencies. The P(Y) codes are intended for exclusive use by the military. Each P(Y) code repeats on a 7-day cycle and modulates both the L1 and L2 carrier frequencies at a 10 MHz rate. Transmission of codes on two different carrier frequencies also allows military receivers to estimate atmospheric effects based on the different refractive effects that the atmosphere has on the two different carrier frequencies.
Because of the digital nature of the codes, the replica code signal generated by a GPS receiver and the code signal received by the GPS receiver from one of the GPS satellites can be out of phase by an amount approaching the width of an individual code pulse and still be synchronized at least part of the time. Note that during the time duration of a single chip of the commercial C/A code output by U.S. GPS satellites, light can travel approximately 293 meters. Since the carrier frequency that conveys the code signal is generally much higher than the chip frequency of the code signal (e.g. by three orders of magnitude), the uniform cycles of the carrier frequency can be used to further synchronize the replica code signal with the received code signal and bring the rising (or falling) edges of the replica code signal and received code signal closer together.
When correlating the predicted code and carrier replica signals to the received signals from the satellites, GPS receivers generate error-signals as part of the tracking process. Traditionally, GPS receivers are designed to perform the tracking process internally with code and carrier tracking loop implementations.
In code tracking, a receiver attempts to minimize the correlation error by advancing and delaying the replica code to synchronize the replica code with the received code. A typical carrier loop is designed to zero-out the carrier phase or frequency errors by applying a correction to the carrier replica oscillator to advance or delay the replica code. Specifically, the tracking errors are sent to a numerically controlled oscillator (NCO) to advance or retard the receiver generated replica signals. When the track errors are near zero, the receiver is said to be in “code-lock” or “carrier-lock”. When the signal is in lock, GPS measurements (pseudo-range and delta-range) can be mathematically generated for navigation filter updates.
The GPS system can be used in conjunction with an INS (Inertial Navigation System) so that the two systems complement each other. Accurate GPS data can be used to supplement or correct INS data. For example, Kalman filtering is used in known implementations to harmonize or correct INS measurements with GPS satellite range and range-rate information. On the other hand, the INS system can aid in tracking operations of the GPS system by smoothly providing accurate short-term measurements of acceleration and velocity that can be used to assist or supplement GPS code tracking and carrier tracking. Aid from the INS system can be useful, for example, when GPS tracking is degraded or jeopardized due to GPS signals suffering interference due to jamming and/or other phenomena including multipath reception, physical blocking of the GPS signals (e.g. by a forest canopy or other structure), and so forth. Interference effects can be compounded with Doppler effects, for example varying Doppler effects due to maneuvering of the GPS receiver.
In order to track signals in the line-of-sight (LOS) domain under non-ideal dynamic situations, a receiver needs to be compensated for receiver-satellite relative motion and receiver oscillator errors. Traditionally, most GPS-inertial navigation systems are equipped with inertial-aiding for this compensation. This scheme is called “loosely-coupled” or “tightly-coupled” integration, depending on the bandwidth of inertial-aiding loops and their implementations. Both are based on traditional track loop implementations in receiver architecture. Although they rely on data from the inertial measurement unit to aid or assist the GPS, they provide limited immunity against momentary GPS signal outages or high interference levels (low signal-to-noise ratio) with GPS signals due to the time-delay of aiding data from the inertial system and less than optimal inter-loop implementations.
If the track loop error exceeds a threshold level, the tracking loop can lose its lock on the satellite signal and must “reacquire” the signal. This track-reacquisition process is independently implemented for each channel (e.g. the CA code for each satellite) without using information from other good channels. Furthermore, since a higher signal-to-noise ratio is required for the signal reacquisition process, the signal might not be reacquired without sufficient improvement in conditions adversely affecting GPS lock.
Recently, numerous organizations have actively engaged in development of integration schemes called “ultra-tightly coupled integration” or “deeply-integrated system”. These implementations do not utilize GPS signals in usual sense. The idea is to employ the navigation filter of the INS as a part of the GPS track loop. The navigation filter, provided with high rate IMU (Inertial Measurement Unit) data, can use the raw GPS receiver outputs, in-phase (I) and quadrature-phase (Q) data, and deliver direct aiding data (NCO) to the GPS receiver (e.g., to a numerically controlled oscillator of the receiver) at a track-loop bandwidth by estimating latest GPS receiver antenna position, velocity, and oscillator bias. Since the correlators' I and Q directly reflect the navigation filter measurement residuals, they are the range domain projections of the navigational (position and velocity) errors and receiver clock bias since the previous filter update. Utilizing the characteristics of I and Q, the navigation filter replaces the conventional track-loop filters. The existing approaches of this ultra-tight system implementation can be implemented via either a) single step, centralized non-linear filtering or b) two step, cascaded or federated filtering.
The centralized filtering combines correlation-track and navigation filters. The navigation filter essentially is part of the signal error track loops providing the latest corrections for pseudo-ranges and pseudo-range rates of all GPS satellites in view of the GPS receiver. This non-linear Kalman filtering approach includes a high computational burden because the filter needs to be updated at a high frequency (e.g. 50 Hz).
The cascaded filtering approach uses two independent filters, a pre-filter and a navigation filter. The pre-filter estimates GPS carrier and code phase errors at high frequency (50 Hz). To follow high dynamics of the errors, their rates and accelerations also need to be estimated. The estimates of the code-phase errors and carrier-phase error rates are then used as measurement residuals in the navigation filter at a slow rate (e.g., 1 Hz). In military applications a pre-filter is independently implemented for each satellite and both L1/L2 carrier frequencies. This large number of the pre-filters (two per satellite) also results in a high computational burden.
In addition, both the cascaded filtering and the centralized filtering approaches require estimation of ionospheric delays to improve the positional accuracy. These delays are usually filtered using dual-frequency measurements (e.g. with respect to the different carrier frequencies L1 and L2 and their different refractive properties), which requires another independent filter for each satellite.